Purpose:
To analyze the combined influence of IOP and age on the risk of undetected glaucoma.

Methods:
A large population screening investigation was performed in Sweden in the years 1992-1997 in order to identify patients with undetected glaucoma for possible inclusion in the Early Manifest Glaucoma Trial. All residents 55 to 79 years of age in the city of Malmö (n=275,000) were invited, with the exception of those who had visited the Malmö University Ophthalmology department within one year prior to the screening. Visual acuity and intraocular pressure were measured, one monoscopic colour photograph of the optic disc was obtained in each eye, and family history was recorded. Subjects with suspect glaucoma or a positive family history were referred for one or more follow-up visits in order to establish or reject a diagnosis of glaucoma. The full eye examinations included visual field testing using the Humphrey 24-2 Full-Threshold program. We calculated the risk of undetected glaucoma per eye in groups combining intraocular pressure (IOP) (5 mmHg intervals) and age (5 year intervals).

Results:
The total number of subjects screened were 32,918 (77.5% of those invited). We found 545 eyes in 406 subjects with previously undiagnosed glaucomatous eyes. The risk of undetected glaucoma per eye increased linearly with age from 0.41% in the youngest age group (55-59 years), to 1.93% in the oldest age group (75-79 years). The risk increased exponentially with IOP from 0.19% at IOP 10-14 mmHg to 57.14 mmHg at IOP >35mmHg. In most IOP groups risk increased with increasing age. Thus in the 10-14 mmHg IOP group from 0.11% to 0.49% and in the 25-29 mmHg group from 4.55% to 18.18% in the youngest and oldest age groups, respectively.

Conclusions:
IOP influences glaucoma risk more than age. The risk increased exponentially with higher IOP and linearly with older age. Combining age and IOP gives better risk estimates than looking at each factor separately.